Structures and machinery components subjected to dynamic loads are prone to experience the formation of cracks and their growth in accordance with the time that the elements are withstanding those loads, also called fatigue loads.
The formation of a crack and its propagation imply a progressive decrease of the strength of the structural element or machinery component which cannot longer work in the intended way for which it was designed and, after a certain time suffering the effect of the fatigue, the residual strength of the structural element is reduced, to the extent that a failure is reached. Hence, it is essential to be able to predict precisely and in real time the decay rate of the residual strength and the remaining life of the structural element.
Fracture mechanics is a branch of science concerned with the study of the propagation of cracks in materials. This field uses methods of analytical solid mechanics to obtain the driving force on a crack and those of experimental solid mechanics to characterize the material resistance to fracture. This field of mechanics helps to predict the service life of structures and machinery components. Applied mechanics covers the analysis of crack tip stress fields as well as the elastic and plastic deformations of the material in the vicinity of the crack. Material science concerns itself with the fracture processes on the scale of atoms and dislocations in the form of impurities and grains.
In order to make a successful use of fracture mechanics in an engineering application, it is essential to have some knowledge of the total technical field.
Fatigue is the weakening of a material caused by repeatedly applied loads. It is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading.
Fatigue failure can occur if the applied load produces an increase in the stress in a point or a zone of the material, with local values exceeding the elastic limit always as a result of the presence of micro-cracks, micro-cavities, local yielding, etc. If the stress is static, the local yielding and the redistribution of the stress onto the surrounding material do not generate any critical condition and the material reaches failure only under considerably higher loads. On the opposite, in the case of dynamic and cyclic loads, the repeated application of the stress leads to the crack propagation until, eventually, the condition of failure is reached and the structural element breaks.
The nominal maximum stress values that cause such damage may be much less than the strength of the material typically quoted as the ultimate stress limit.
Unlike metallic structures and machinery components, historically, concrete structures have not been designed to be subjected to fatigue stresses due to loading cycles along their service lives, which condition their geometry and dimensions. For these scenarios a steel structure was usually chosen or a structure made from another material that was able to resist fatigue.
Generally, concrete was used for structures that, even being able to punctually resist loading cycles, were subjected to other static actions much more determining for their design and dimensioning than fatigue.
Material science has allowed to develop concrete formulas that reach higher strength levels than before and, therefore, civil engineering, in order to extend the application fields of the material, has made possible to use concrete in the design of structures more exigent in terms of fatigue.
Nevertheless, the current codes an international standards, that rule the design of concrete structures, are still vague in relation to the fatigue analysis and do not allow to reach the detail level of assessment necessary to optimize the design of structures subjected to cyclic loads, whose dimensions are conditioned by the concrete fatigue strength.
The standard formulation is limited to derive a concrete fatigue strength from the value of the compressive strength of the material, disregarding many phenomena that significantly influence the concrete fatigue strength. The uncertainties associated to the lack of knowledge about those effects are covered, in the codes and standards, by applying several strong reduction coefficients in order to be always on the safe side.
In the most usual cases, where the structure design and its dimensions are not conditioned by the cyclic loads that will affect the structure along its service life, the standard procedure is considered enough to verify that the structure will not suffer fatigue failure.
Nevertheless, in those special cases where the structure is subjected to cyclic loads in such amount that its design and dimensions are conditioned by the fatigue strength of the concrete, the fact of applying the standard formulation implies the oversizing of the structure and the increase of its cost.